A microbial fuel cell (MFCs) is a type of bioelectrochemical system (BES) where current is generated by bacteria from the oxidation of organic and inorganic matter. Compounds are reduced at the cathode, such as oxygen or protons, through inorganic catalysts or bacteria and electrical power is generated in an MFC.
Ordinary fuel cells (i.e. PEM hydrogen fuel cells) can be linked in series to increase voltage, but when MFCs are electrically linked in series there is a voltage reversal, and the power output from the stack (multiple cells in series) is not consistently increased in proportion to the number of cells. This electrical linking of MFC reactors in series has limited applications where higher voltages are desired.
Hydrogen gas is generated by addition of voltage to an MFC, in which case the system is termed a microbial electrolysis cell (MEC). Exoelectrogens are used to drive electrochemical H2 production in an MEC. However, the potential generated by substrate oxidation (−0.30 V vs. NHE; 1 g/L acetate; pH 7) is not sufficient to drive H2 evolution (−0.41 V vs. NHE at pH 7). Thus, additional energy (˜0.11 V in theory) is needed to overcome this thermodynamic threshold, and an external voltage of >0.4 V is typically applied to MECs. This additional energy could be provided by, for instance, a renewable source of energy, such as solar, wind, or waste organic matter. However, no method or device is believed to have been developed prior to the present invention to directly achieve H2 production in one biotic process without an external voltage supply.
Reverse electrodialysis (RED) holds great promise as a method for generating electricity from the salinity gradient between seawater and river water. RED systems are built as stacks of alternating cation- and anion-exchange membranes situated between two electrodes. When seawater and river water are provided into the RED stack, counter-ions (selected ions) to the membranes are driven from seawater to river water due to the salinity difference, creating an electric potential across the ion-exchange membrane. A salinity ratio of 50 between seawater and river water can theoretically create 0.155 V (open circuit) per pair of anion- and cation-exchange membranes.
Reverse electrodialysis (RED) is a power generation system based on the salinity-driven electromotive force, especially between salty sea water and fresh river water (Huang, et al., 2006). The concentration difference across the ion-exchange membrane drives the ionic transport in the system, and this driving force can be quantified by Eq. (1) as electromotive force (Bard, et al., 2001).
                                        Δϕ                          =                              RT            zF                    ⁡                      [                                                            t                  counter                                ⁢                                  ln                  ⁡                                      (                                                                  a                                                  counter                          ,                          high                                                                                            a                                                  counter                          ,                          low                                                                                      )                                                              -                                                t                  co                                ⁢                                  ln                  ⁡                                      (                                                                  a                                                  co                          ,                          high                                                                                            a                                                  co                          ,                          low                                                                                      )                                                                        ]                                              (        1        )            where Δφ is the electromotive force driven by the concentration difference, t is the transport number (defined as the fractional contribution of the ionic flux to the current density in the membrane), R is the gas constant, T is the absolute temperature, z is the ionic charge, f is the activity coefficient, and c is the ionic concentration. The subscripts high and low mean the high- and low-concentration cells, respectively. Also, the subscripts counter and co denote the counter- and co-ions to the membrane, respectively. Note that the counter- and co-ions are selected and excluded ions by the ion-exchange membrane; for instance, with a cation-exchange membrane, sodium or magnesium ions are counter-ions, while chloride or sulfate ions are co-ions, and vice versa with an anion-exchange membrane.